Combustion plants emit large amount of flue gas containing fly ash and harmful gasses, such as SO2. Therefore, to avoid undesired impact on the environment, combustion plants usually have installed flue gas desulfurization systems to reduce SO2 emission and units reducing the particulate emission.
A particular attractive way of removing particles as well as acidic components from a flue gas is provided by a spray dryer absorber (SDA). In an SDA acidic gasses are absorbed in an absorbent sprayed into the flue gas. Typically, the flue gas is introduced into a spray dryer absorption chamber and contacted with a fine spray of absorbent slurry, such as an aqueous suspension of burned lime. A significant part of the acidic components in the flue gas are rapidly absorbed into the alkaline droplets and water is evaporated simultaneously. The gas distribution, slurry flow rate and droplet size are controlled such that the droplets are dried to a fine powder.
U.S. Pat. No. 4,279,873 (Niro) discloses an SDA of the type disclosed above, which further have been improved by partly recycling of the fine powder produced in the chamber of the SDA. The recycling results in the use of less absorbent because unreacted alkaline absorbent, such as Ca(OH)2, remains in the fine powder.
Poor quality coal and other fossil combustibles generate significant amounts of ash as a by-product of combustion. This ash typically is collected at the bottom of the furnace but significant quantities as fly ash can become entrained in the flue gas stream exiting the furnace.
The fly ash is highly abrasive and can abrade ducting, guide vanes, dampers and other components of the SDA. The ash particles collide with the apparatus components and causes extensive surface abrasion. In advanced stages of abrasion the components get perforated and may fail once they lose their structural integrity. Such abrasion shortens the service-life of the SDA components. When damaged components have to be replaced, the plant, e.g. a power station, has to be shut down. The resulting costs are not only the costs of replacing the components but also the cost of stoppage of power production.
To reduce the wear of sensitive parts of the gas disperser a pre-collector to filter out fly ash may be required. U.S. Pat. No. 5,738,711 suggests using a hopper with a deflector and a screening arrangement to separate fly ash from the flue gas exiting a furnace. Others have suggested the use of electrostatic precipitators, cyclones or bag filters to reduce the content of fly ash in the flue gas.
U.S. Pat. No. 4,530,822 shows some embodiments of an SDA and the use of different atomizing means such as a rotary atomizer wheel or nozzles.
The present invention is based on the assumption that small amounts of fly ash, such as below 20 g/Nm3, are acceptable. However, beyond that the fly ash creates too much wear on the gas disperser which feeds the flue gas into the SDA. Hitherto, the excessive amounts of fly ash have been separated out in external pre-collectors, which, however, complicate the entire process and contribute to the total costs of the plant. It is the aim of the present invention to substantially reduce the cost of the entire SDA plant for treating flue gas having a high content of entrained particles. Lower cost of the SDA system may prompt areas rich in coal of a poor quality to control the air pollution by using a SDA system for treating the flue gas. A further aim is to reduce the “foot print” of the plant, i.e. the ground area needed for erecting the plant.